perf(maintenance): back DeletionBitmap with a Roaring bitmap

[mosuka]

Closes #684 (cross-cutting data-structure rewrite, umbrella #537).

Problem

DeletionBitmap (laurus/src/maintenance/deletion.rs) stored deleted ids as RwLock<ahash::AHashSet<u64>>, and the .delmap (v3) wrote them as a raw u64 list. For the dense deletion sets that accumulate over a segment's life this is huge — a 10M-doc / 10%-deleted segment is ~8 MB on disk (and a multi-MB hash table in RAM whose is_deleted probes miss cache). This same bitmap is consumed by both lexical (filter_deleted_soa) and every vector index (HNSW / Flat / IVF is_deleted), which are per-doc / per-neighbour hot paths.

Change

Swap the internal representation to roaring::RoaringTreemap (a dependency since #578; doc ids are the global u64 space):

• is_deleted becomes a branch-light bit test that stays cache-resident for dense sets; delete_document uses RoaringTreemap::insert's newly-added bool; get_deleted_docs returns ascending ids; memory_usage reports the Roaring serialized_size.
• .delmap gains a v4 payload (RoaringTreemap::serialize_into); the reader still loads v1/v2/v3 (raw id list) for backward compatibility.
• The public API is unchanged (is_deleted / delete_document / get_deleted_docs / … keep their signatures), so the ~20 lexical/vector consumers benefit with no changes — this single change covers both domains and subsumes the per-domain perf(lexical/index): replace ahash::AHashSet<u64> liveDocs with Roaring/FixedBitSet #541 (lexical) / perf(vector/index): replace DeletionBitmap's AHashSet<u64> with a Roaring bitmap (with optional dense BitVec shard) #625 (vector).

For a dense set, ~125 KB stays L2-resident (bit test) vs a multi-MB hash table that misses cache on each probe — the RAM/on-disk drop is >50×.

Locking decision

Kept as RwLock<RoaringTreemap> (in-place O(1) delete). An ArcSwap + RCU model would clone the whole bitmap on every delete_document — O(D²) over a segment's deletions (which are applied one-at-a-time at upsert/commit and accumulate to hundreds of thousands) — so it would badly regress the write path. Truly lock-free is_deleted (consumer snapshot-once, or a batched-delete API) is left as a follow-up.

Verification

• cargo build (full workspace + bindings) ✅
• cargo clippy --all-targets -- -D warnings — zero warnings ✅
• cargo fmt --check — clean ✅
• cargo test -p laurus --lib — 1104 passed / 0 failed (+2); cargo test --workspace — exit 0, 51 binaries ✅
• markdownlint-cli2 — 0 errors; docs (en + ja) updated ✅

New unit tests: v4 round-trip (write → read yields the same set/order/metadata) and v3 backward-read (a hand-written v3 payload still loads correctly). The existing lexical/vector deletion tests pass unchanged.

Note on benchmarking

The plan called for an is_deleted micro-bench, but DeletionBitmap lives in a private module (maintenance), unreachable from an external Criterion bench — and making it pub purely for a bench contradicts this PR's "no public API change" goal. The RAM/on-disk win is structural (Roaring vs a raw u64 list), and the dense-set is_deleted speedup is the well-known cache-resident-bitmap vs cache-missing-hash-table property; correctness is covered by the tests above.

Follow-up

• Lock-free is_deleted (consumer snapshot-once or batched delete) — avoids the COW-per-delete regression that naive ArcSwap would cause.

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